1. Field of the Invention
This invention relates generally to linearization and calibration of electromechanical scanning devices, and more particularly relates to an apparatus for position measurement of a sample scanning stage used with scanning microscopes and surface measurement systems.
2. Description of Related Art
Devices for producing precise linear, two or three dimensional motion have proven to be highly useful in scanning devices. In particular, electromechanical transducers such as piezoelectric ceramic actuators, which expand upon being subjected to an electrical potential, have been used for X-Y-Z positioners in scanning probe microscopes. Such piezoelectric ceramic materials have been combined in laminates, tubes, or stacks, to allow two-dimensional and even three-dimensional motion of the sample stages for such systems.
Piezoelectric ceramic actuators are electromechanical elements that undergo dimensional changes about a poling axis which has been formed in the material during the manufacturing process. When an electrical field is applied to the ceramic, the material generally expands about the poling axis and contracts perpendicular to the poling axis. However, the dimensional response of such piezoelectric materials to an applied voltage is not linear, and such materials commonly display varying degrees of hysteresis, creep, and a variable sensitivity to application of voltage. Hysteresis occurs due to a difference in dimensional changes in response to an applied voltage, depending upon whether the voltage is an increase or decrease from the previous applied voltage. Although the degree of hysteresis and non-linearity of response is less for hard piezoelectric materials (having a Curie temperature above 300.degree. C., and producing smaller displacements) than for soft piezoelectric materials (having a Curie temperature below 200.degree. C., and producing larger displacements), the hard piezoelectric material still typically has a degree of hysteresis on the order of 2%, and a deviation from linearity of about 1%. Creep is a phenomenon of temporary dimensional stabilization which occurs after application of a step change in voltage to cause an initial dimensional change, followed by a gradual, long term, small dimensional change in the direction of the initial change. The amount of creep for a piezoelectric material can range from 1% to 20% of the initial dimensional response over a period of about 10 to 100 seconds.
Scanning probe microscopes such as scanning force microscopes, also known as atomic force microscopes, are useful for imaging objects as small as atoms. The scanning force microscope is closely related to the scanning tunneling microscope and the technique of stylus profilometry. In a typical scanning force microscope, a laser beam is directed at a reflective portion of a lever arm carrying a probe so that a vertical movement of a probe following the contours of a specimen is amplified into a relatively larger deflection of the light beam. The deflection of the laser beam is typically monitored by a photodetector array in the optical path of the deflected laser beam, and the sample is mounted on a sample stage moveable in minute distances in three dimensions so that the sample can be raster scanned while the vertical positioning of the probe relative to the surface of the sample is maintained substantially constant by a feedback loop with the photodetector controlling the vertical positioning of the sample. Such scanning force microscopes are useful for imaging a sample which is moved in three dimensions while the sensor head is stationary and separate from the scanning assembly moving the sample. Alternative constructions in which the sample is held stationary while the probe is moved may also be used to accomplish essentially the same results.
Scanning force microscope images can be severely distorted due to problems of hysteresis, creep, and generally non-linear response of piezoelectric materials used in scanning devices for such microscopes. In view of the high resolution and positioning accuracy required to avoid distortions in scanning force microscope imaging, it would be desirable to provide an electromechanical scanning apparatus which insures precise translational motion of the scanning device and accurate measurement of the position of the probe relative to the sample.
Distortions in scanner displacement of an X-Y-Z scanner have been typically corrected by closed loop feedback correction or postimaging software, based upon determination of corrected (x,y) positions according to a formula with a number of variables, and various strategies for interpolation, or based upon measurements of the actual scanner displacement. Correction by postimaging software can be time consuming and require a high utilization of computing resources; and interpolation errors in the process can blur and distort the image. Linear-variable differential transformer, optical interferometry, capacitance, and optical beam position sensing methods have also been used for measuring actual scanner displacement. Interferometry has proven to be very accurate, but results in a periodic output function, and is complex to implement. A photoelectric differencing system, with a predetermined dynamic range, has also proven useful, but has a limited resolution capability. Strain gauges that change their electrical resistance with a change in length are sensitive indicators that can be bonded directly to piezoelectric actuators to give an indication of localized extension of the actuator, which can in turn be used to extrapolate approximate displacement of a stage. However, mounting the strain gauges directly on the ceramic does not work well when the scan range is greater than approximately 1 micron (.mu.).
It would be desirable to provide an improved system for linearizing and calibrating non-linear electromechanical scanning devices having greater than 1.mu. of extension, with improved linearity over conventional methods, thereby reducing problems of hysteresis, creep and non-linear displacement responses. It would also be desirable to provide such an improved system that is small and relatively inexpensive to manufacture. The present invention meets these needs.